(11β,16α,17α)-9-Chloro-17-(2,2-dioxido-5H-1,2-oxathiol-4-yl)-17-[(2-furanylcarbonyl)oxy]-11-hydroxy-16-methyl-androsta-1,4-dien-3-one (Mometasone EP Impurity B) is an impurity of Mometasone Furoate, a topical corticosteroid used as an anti-inflammatory.